How much of “Doctor Who” might really be possible?

Science shows why Doctor Who is so special.

As Doctor Who’s 50th anniversary looms, time travel is everywhere – on the screen, at least. Famously, the Doctor can whizz through the years using a “dimensionally transcendental” machine, the TARDIS, and make changes to the past as and when he likes. But what is time travel – and how much of “Doctor Who” might really be possible?

A handy definition of time travel comes from philosopher David Lewis. Lewis says time travel involves a journey having different durations viewed from outside (in “external time”) or from inside (in “personal time”). Suppose you spend five minutes travelling aboard your machine, as measured by (e.g.) your watch and your memories. On arrival, you find 150 years have elapsed in the outside world. Congratulations, you have time-travelled. Five minutes of your personal time has covered 150 years of external time.

Odd as this sounds, Einstein’s theory of Special Relativity introduced such possibilities to physics in 1905. The theory says: the duration of a process varies with the relative velocity of the observer. The closer that relative velocity gets to the speed of light, the longer the travelling process takes.

Suppose you want to see the Earth a billion years hence, but worry you have only about 50 personal years left. Special Relativity specifies that if you travel very close to the speed of light relative to the Earth, your 50 personal years can cover one billion Earth years.

In backward time travel, personal and external time differ in direction, so journeys end in external time before, not after, they begin; you spend five personal minutes travelling 150 years into the external past. General Relativity suggests that the universe is essentially curved spacetime, which might allow such divergences of external and personal time.

Relativity treats space and time as aspects of a single entity: “spacetime”. One of the more remarkable features of General Relativity is that it allows time and space axes to be interchanged, so one observer’s space axis can be another observer’s time axis.

In 1949, Austrian mathematician Kurt Gödel used General Relativity to describe a universe where intrepid voyagers can go anywhere in (past or future) time without travelling faster than light. Gödel’s universe has no boundaries in space or time, and all the matter in it rotates. But our finite, non-rotating universe is not Gödel’s. Despair not though – simply spin an ultradense, very (maybe infinitely) long cylinder very fast. Spacetime should curve around the cylinder so the direction of the local future partially points into the external past. Such devices are called “Tipler Cylinders”, after physicist Frank Tipler.

Better yet, quantum theory suggests that “wormhole” connections between different spacetime points spontaneously form and break all the time. The chances are that natural wormholes are tiny - vastly smaller even than an electron, (and a billion trillion electrons can fit in a teaspoon). But you could perhaps find (or create) a wormhole big enough and durable enough to let you slip through into the past. Difficult, but theoretically possible.

No, you can’t kill your physics teacher

So perhaps you could travel into the past. But what about paradoxes? What is to stop you assassinating your grandfather or yourself as infants? One answer says: logical consistency.

Classical logic says you cannot consistently kill in infancy someone who achieves adulthood. But, Lewis says, time travel need not involve doing the logically impossible – provided travellers’ actions in the past are consistent with the history whence they come. So you could try killing your baby grandfather, but something would foil you – you would sneeze, or your gun would jam. Lewisian time travel is therefore (classically) consistent, but might look very strange, since seemingly possible actions (like shooting an unprotected infant) would prove impossible.

Another view says that backward time travel requires many worlds – that is, many different but equally real versions of physical reality. Physicist David Deutsch and philosopher Michael Lockwood argue that time travel must involve inter-world travel. If you travel backwards in time, you must arrive in a history different from your native one and so would be quite unfettered by your past once you get there. You could even kill this other history’s counterparts of your grandfather and yourself.

Both these concepts of backwards time travel may disappoint anyone wanting to change the “one and only” past. Conventional logic says time travellers would either help make history what it was (Lewis) or create a different history (Deutsch/Lockwood). However, quantum logic might let travellers change the actual (one-and-only) past.

Suppose we hold that quantum measurements determine (or change) quantities measured, even if those quantities lie in the past. Someone could travel back and “observe” history turning out differently from how it originally was, thereby retrospectively making actuality different from what it had been. What would happen to travellers who rebooted history is not clear, but this model seems closer to the time travel familiar from “Doctor Who” and other fictions. Beware, though, because quantum theory allows no predicting, and still less controlling, of the outcomes of changing the past. There would be no way to foresee the effect you would have on the present.

So classical logic, General Relativity and quantum theory all seem to permit time travel. Classical logic plus General Relativity suggest backward travellers face weird consistency constraints. Many-worlds travellers face no constraints, but get displaced into different histories. Quantum-logic travellers could change the (one and only) past without constraints, but they couldn’t predict or control what they would get.

So far, however, it seems only the Doctor knows how to change the past at will.

Alasdair Richmond received funding from the Arts and Humanities Research Council, to fund one semester of a two-semester leave period that covered the academic year 2008-2009.

The Conversation

This article was originally published at The Conversation. Read the original article.

The eleven incarnations of the Doctor. Image: BBC/Matt Burlem

Alasdair Richmond is a Senior Lecturer in Philosophy at the University of Edinburgh.

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Katy Perry’s new song is not so much Chained to the Rhythm as Chained to a Black Mirror episode

The video for “Chained to the Rhythm” is overwhelmingly pastel and batshit crazy. Watch out, this satire is sharp!

If you’ve tuned into the radio in the last month, you might have heard Katy Perry’s new song, “Chained to the Rhythm”, a blandly hypnotic single that’s quietly, creepingly irresistible.

If you’re a really attuned listener, you might have noticed that the lyrics of this song explore that very same atmosphere. “Are we crazy?” Perry sings, “Living our lives through a lens?”

Trapped in our white picket fence
Like ornaments
So comfortable, we’re living in a bubble, bubble
So comfortable, we cannot see the trouble, trouble
Aren’t you lonely?
Up there in utopia
Where nothing will ever be enough
Happily numb

The chorus muses that we all “think we’re free” but are, in fact, “stumbling around like a wasted zombie, yeah.” It’s a swipe (hehe) at social media, Instagram culture, online dating, whatever. As we all know, modern technology is Bad, people who take photos aren’t enjoying the moment, and glimpses other people’s Perfect Lives leave us lonely and empty. Kids these days just don’t feel anything any more!!!

The video for this new song was released today, and it’s set in a (get this) METAPHORICAL AMUSEMENT PARK. Not since Banky’s Dismaland have we seen such cutting satire of modern life. Walk with me, through Katy Perry’s OBLIVIA.

Yes, the park is literally called Oblivia. Get it? It sounds fun but it’s about oblivion, the state of being unaware or unconscious, i.e. the state we’re all living in, all the time, because phones. (I also personally hope it’s a nod to Staffordshire’s own Oblivion, but cannot confirm if Katy Perry has ever been on the Alton Towers classic steel roller coaster.)

The symbol of the park is a spaced-out gerbil thing, because, aren’t we all caged little hairy beings in our own hamster wheels?! Can’t someone get us off this never-ending rat race?!

We follow Katy as she explores the park – her wide eyes take in every ride, while her peers are unable to look past the giant iPads pressed against their noses.


You, a mindless drone: *takes selfies with an iPad*
Katy Perry, a smart, engaged person: *looks around with actual human eyes, stops to smell the roses*

She walks past rides, and stops to smell the roses – and the pastel-perfect world is injected with a dose of bright red reality when she pricks her finger on a thorn. Cause that’s what life really is, kids! Risk! At least she FEELS SOMETHING.


More like the not-so-great American Dream, am I right?!

So Katy (wait, “Rose”, apparently) takes her seat on her first ride – the LOVE ME ride. Heteronormative couples take their seats against either a blue heart or a pink one, before being whizzed through a tunnel of Facebook reaction icons.

Is this a comment on social media sexism, or a hint that Rose is just too damn human for your validation station? Who knows! All we can say for sure is that Katy Perry has definitely seen the Black Mirror episode “Nosedive”:

Now, we see a whole bunch of other rides.


Wait time: um, forever, because the human condition is now one of permanent stasis and unsatisfied desires, duh.

No Place Like Home is decorated with travel stamps and catapults two of the only black people in the video out of the park. A searing comment on anti-immigrant rhetoric/racism? Uh, maybe?

Meanwhile, Bombs Away shoots you around like you’re in a nuclear missile.


War: also bad.

Then everyone goes and takes a long drink of fire water (?!?!) at Inferno H2O (?!?!) which is also a gas station. Is this about polluted water or petrol companies or… drugs? Or are we just so commercialised even fire and water are paid-for privileges? I literally don’t know.

Anyway, Now it’s time for the NUCLEAR FAMILY SHOW, in 3D, no less. Rose is last to put her glasses on because, guess what? She’s not a robot. The show includes your typical 1950s family ironing and shit, while hamsters on wheels run on the TV. Then we see people in the rest of theme park running on similar wheels. Watch out! That satire is sharp.

Skip Marley appears on the TV with his message of “break down the walls to connect, inspire”, but no one seems to notice accept Rose, and soon becomes trapped in their dance of distraction.


Rose despairs amidst the choreography of compliance.

Wow, if that didn’t make you think, are you even human? Truly?

In many ways – this is the Platonic ideal of Katy Perry videos: overwhelmingly pastel, batshit crazy, the campest of camp, yet somehow walking the fine line between self-ridicule and terrifying sincerity. It might be totally stupid, but it’s somehow still irresistible.

But then I would say that. I’m a mindless drone, stumbling around like a wasted zombie, injecting pop culture like a prescription sedative.

I’m chained…………. to the rhythm.

Anna Leszkiewicz is a pop culture writer at the New Statesman.